Axial skeleton

The axial skeleton is a major division of the human skeletal system, consisting of 80 bones. It includes the bones of the head, neck, and trunk. Specifically, the axial skeleton comprises the skull, vertebral column, ribs, and sternum. This central framework supports and protects the brain, spinal cord, and vital organs within the thorax. The skull includes cranial and facial bones, the vertebral column consists of vertebrae, and the thoracic cage includes the ribs and sternum. The axial skeleton is essential for maintaining the body's structural integrity and providing attachment points for muscles.

Location

The axial skeleton is located along the central axis of the body. It includes:

• Skull: Located in the head, consisting of cranial and facial bones.
• Vertebral Column: Extending from the base of the skull to the pelvis, forming the backbone.
• Thoracic Cage: Located in the chest, comprising the ribs and sternum, encasing and protecting vital organs like the heart and lungs.

Structure

The axial skeleton forms the central framework of the human body, supporting and protecting the organs of the head, neck, and trunk. It consists of three major components: the skull, vertebral column, and thoracic cage.

Skull

The skull is composed of 22 bones, divided into the cranial bones and the facial bones. Cranial Bones The cranial bones form the protective case around the brain and include:

• Frontal Bone: Forms the forehead and the upper part of the eye sockets.
• Parietal Bones (2): Form the sides and roof of the cranial cavity.
• Temporal Bones (2): Located at the sides and base of the skull, housing the structures of the inner and middle ear.
• Occipital Bone: Forms the back and base of the skull, containing the foramen magnum through which the spinal cord passes.
• Sphenoid Bone: Located at the base of the skull, contributing to the floor of the cranial cavity.
• Ethmoid Bone: Located between the eyes, forming part of the nasal cavity and the orbits.

Facial Bones The facial bones form the structure of the face and include:

• Nasal Bones (2): Form the bridge of the nose.
• Maxillae (2): Form the upper jaw and part of the orbits.
• Zygomatic Bones (2): Form the cheekbones and part of the orbits.
• Mandible: The lower jawbone, the only movable bone of the skull.
• Lacrimal Bones (2): Small bones forming part of the medial walls of the orbits.
• Palatine Bones (2): Form part of the hard palate of the mouth and the floor of the nasal cavity.
• Inferior Nasal Conchae (2): Form part of the lateral walls of the nasal cavity.
• Vomer: Forms the lower part of the nasal septum.

Hyoid Bone Although not directly attached to the skull, the hyoid bone is located in the neck and supports the tongue and its muscles.

Vertebral Column

The vertebral column, or spine, is composed of 33 vertebrae, divided into five regions: Cervical Vertebrae (7) Located in the neck, these vertebrae are numbered C1 to C7. The first cervical vertebra (atlas) supports the skull, and the second cervical vertebra (axis) allows for head rotation. Thoracic Vertebrae (12) Located in the upper and mid-back, these vertebrae are numbered T1 to T12 and articulate with the ribs. Lumbar Vertebrae (5) Located in the lower back, these vertebrae are numbered L1 to L5 and are the largest and strongest vertebrae. Sacral Vertebrae (5, fused) These vertebrae are fused to form the sacrum, which connects the spine to the pelvic girdle. Coccygeal Vertebrae (4, fused) These vertebrae are fused to form the coccyx, or tailbone.

Thoracic Cage

The thoracic cage, or ribcage, is composed of the ribs, sternum, and thoracic vertebrae. Ribs (24) There are 12 pairs of ribs, each articulating with the thoracic vertebrae:

• True Ribs (1-7): Directly attached to the sternum via costal cartilage.
• False Ribs (8-10): Indirectly attached to the sternum via the cartilage of the ribs above them.
• Floating Ribs (11-12): Not attached to the sternum.

Sternum The sternum, or breastbone, is a flat bone located in the center of the chest and consists of three parts:

• Manubrium: The upper part of the sternum, articulating with the clavicles and the first pair of ribs.
• Body: The longest part of the sternum, articulating with the costal cartilages of the second to seventh ribs.
• Xiphoid Process: The small, cartilaginous lower tip of the sternum, which ossifies in adulthood.

Function

The axial skeleton plays several crucial roles in the human body, contributing to its overall functionality and health.

Structural Support

Framework for the Body The axial skeleton provides the primary framework that supports the body. It forms the central axis around which other parts of the skeleton, such as the appendicular skeleton (limbs and girdles), are organized. This structural support maintains the body’s shape and posture. Support for Soft Tissues The bones of the axial skeleton serve as attachment points for muscles, ligaments, and tendons, supporting soft tissues and ensuring they remain in proper anatomical positions.

Protection

Protection of the Brain The skull encases and protects the brain from mechanical damage. The cranial bones form a rigid barrier that shields the brain from impacts and injuries. Protection of the Spinal Cord The vertebral column houses and protects the spinal cord within its vertebral canal. The vertebrae provide a sturdy enclosure that guards the spinal cord against physical damage while allowing flexibility and movement. Protection of Thoracic Organs The thoracic cage, composed of the ribs and sternum, protects vital organs within the chest, such as the heart, lungs, and major blood vessels. The ribs form a protective barrier that absorbs impacts and shields these organs from injury.

Movement Facilitation

Muscle Attachment Points The bones of the axial skeleton serve as attachment points for various muscles. These muscles, when contracted, enable movements such as bending, twisting, and extending the spine. The ribcage also provides attachment points for muscles involved in respiration. Flexibility and Mobility The vertebral column’s segmented structure, with intervertebral discs between each vertebra, allows for flexibility and a range of motions. This flexibility is crucial for activities such as bending and rotating the torso, providing balance and mobility.

Hematopoiesis

Production of Blood Cells Red bone marrow, found within the cavities of certain bones in the axial skeleton, is responsible for hematopoiesis. This process produces red blood cells, white blood cells, and platelets, which are essential for carrying oxygen, fighting infections, and blood clotting, respectively.^[5]

Mineral Storage

Reservoir for Minerals The axial skeleton acts as a storage site for essential minerals, primarily calcium and phosphorus. These minerals are critical for various physiological processes, including bone strength, nerve transmission, and muscle contraction.^[3] The bone matrix can release these minerals into the bloodstream as needed to maintain homeostasis.

Sound Transmission

Hearing Mechanism The bones of the middle ear, known as the auditory ossicles (malleus, incus, and stapes), are located within the temporal bones of the skull. These tiny bones transmit sound vibrations from the eardrum to the inner ear, facilitating hearing.

Respiratory Function

Assistance in Breathing The thoracic cage plays a crucial role in respiration.^[1] The ribs and sternum move during inhalation and exhalation, expanding and contracting the chest cavity. This movement allows the lungs to inflate and deflate, enabling efficient breathing.

Clinical Significance

The axial skeleton holds immense clinical significance due to its critical roles in providing structural support, protecting vital organs, facilitating movement, and more. Disorders or injuries affecting the axial skeleton can have profound impacts on an individual's health and quality of life.^[8]

Spinal Disorders

The vertebral column is susceptible to a range of conditions that can cause pain, disability, and neurological deficits. Common spinal disorders include:

• Herniated Discs: The intervertebral discs can bulge or rupture, compressing nearby nerves and causing pain, numbness, or weakness.
• Scoliosis: An abnormal lateral curvature of the spine that can lead to uneven posture and, in severe cases, respiratory and cardiovascular issues.
• Osteoarthritis: Degenerative changes in the spine can lead to pain and stiffness, particularly in the cervical and lumbar regions.^[6]

Fractures

Fractures of the axial skeleton, such as skull fractures, vertebral fractures, and rib fractures, are serious injuries that can result from trauma or underlying conditions like osteoporosis.

• Skull Fractures: These can cause brain injury, leading to long-term neurological impairments or life-threatening complications.
• Vertebral Fractures: These can compromise spinal cord integrity, potentially causing paralysis or chronic pain.
• Rib Fractures: These can impair respiratory function and are often associated with significant pain.^[4]

Osteoporosis

Osteoporosis is a condition characterized by reduced bone density and increased fragility, predominantly affecting the axial skeleton.^[2] Vertebral compression fractures are common in osteoporosis, leading to chronic pain, reduced mobility, and a decrease in quality of life.

Infections and Inflammatory Conditions

Infections such as osteomyelitis can affect the bones of the axial skeleton, leading to severe complications if not treated promptly. Conditions like ankylosing spondylitis, a type of inflammatory arthritis, primarily affect the spine and sacroiliac joints, causing pain and reduced flexibility.

Congenital Anomalies

Congenital anomalies such as spina bifida, a defect in the development of the vertebral column, can result in neurological impairments and require surgical intervention and long-term management.

Cancer

The axial skeleton can be affected by primary bone cancers, such as osteosarcoma, or metastatic cancers that spread from other parts of the body. Vertebral metastases can cause spinal cord compression, necessitating urgent medical or surgical treatment.